1. Field of the Invention
The present invention relates to a device for purifying an exhaust gas from an internal combustion engine. More particularly, the present invention relates to a device for purifying hydrocarbon (HC) components in an exhaust gas in a gasoline internal combustion engine, which are generated when an engine in a cold condition is started.
2. Description of Related Art
A catalytic converter is usually arranged in an exhaust pipe of an internal combustion engine, so that exhaust gas contacts the catalyst layers of the catalytic converter. As a result, unburnt components, such as hydrocarbon (HC), are purified. However, in order to obtain such an operation, the catalyst must be in an activated condition, i.e., the catalyst must be in a desirable temperature range. Thus, when an engine in a cold condition is started, the exhaust gas purification can not be obtained until the temperature of the catalyst is increased to the desired value.
In order to obviate this problem, Japanese Unexamined Patent Publication No. 6-212951 proposes a solution wherein an adsorption unit is arranged in an exhaust manifold in such a manner that the HC adsorption unit faces an exhaust port of the engine. Due to the arrangement of the adsorption unit with respect to the exhaust port, a straight flow of exhaust gas from the exhaust port first contacts the adsorption unit, so that hydrocarbon components, in the exhaust gas at a low temperature, generated just after the engine is started are captured and temporarily held by the adsorption unit. An increase in the temperature of the exhaust gas causes the hydrocarbon components to be released from the adsorption unit and entrained in the flow of the exhaust gas. The released hydrocarbon components in the flow of the exhaust gas are purified at a catalytic converter located at a downstream position, which is now in an activated condition.
In the above mentioned patent, the adsorption unit is constructed by a casing which is opened only at a side facing the exhaust port, and a carrier body including HC adsorbent housed in the casing. Due to this one end closed structure, the straight flow of the exhaust gas contacting with the HC adsorbent is prevented from being passed through the layer of the HC adsorbent, so that an increase in the temperature to the HC release temperature (about 100.degree. C.) after the start of the engine occurs relatively slowly. As a result, the HC components generated prior to the completion of the activation of the catalytic converter are held in the adsorption unit, on one hand and, on the other hand, the adsorption unit is prevented from being overheated when the temperature of the exhaust gas is high due to the execution of a high load operation.
However, even in the above mentioned improved system, it may be possible that a premature desorption occurs prior to the activation of the catalyst located downstream from the adsorption unit, which causes the HC components to be exhausted without being purified at the catalytic converter. Thus, there has been a long felt need that the adsorption unit is, for a prolonged time, maintained at a low temperature whereat the HC adsorption capacity is high. Furthermore, in the above mentioned prior art system, an arrangement of the exhaust system is such that the exhaust gas is partly directly directed to the exhaust pipe. In other words, lighter components in the exhaust gas are directed to the exhaust pipe without making contact with the adsorption unit, thereby reducing HC adsorption capacity compared to a construction wherein all of the exhaust gas flows into the HC adsorption unit as disclosed in the Japanese Unexamined Patent Publication No. 2-135126.
Furthermore, it is usual for an internal combustion engine to use a rich air-fuel mixture during a high load operation, which makes the temperature of the exhaust gas low, thereby preventing the catalytic converter from being overheated. However, in order to allow an internal combustion engine to be operated under a theoretical air-fuel ratio, an arrangement is often employed, in a vehicle for use in European countries, wherein a catalytic converter is arranged below a floor of the vehicle, where the temperature of the exhaust gas is relatively low. However, a use of the prior art device (Japanese Unexamined Patent Publication No. 6-212951) in such a type of vehicle will easily cause the temperature of the exhaust gas from an exhaust port to be highly increased, so that the temperature of the adsorption unit is also highly increased, which causes the fine structure of the adsorption unit to be easily damaged. Furthermore, a substantial gas flow is not generated inside the HC adsorption unit, which prevents the HC gas from being fully subjected to a desorption even after the temperature of the adsorbent is higher than a predetermined desorption temperature, resulting in the HC components being carbonized, thereby reducing the HC adsorption capacity.